The present invention relates to a wear-resistant compound roll suitable for hot and cold rolling and a method of producing it, and more particularly to a wear-resistant compound roll having a shell portion made of a sintered material showing excellent wear resistance and toughness, and a method of producing it.
The rolls are required to have roll surfaces suffering from little wear, little surface roughening, little sticking with materials being rolled, less cracks and fractures, etc. For this purpose, cast compound rolls having hard outer surfaces and forged steel rolls having roll body portions hardened by heat treatment, etc. are conventionally used.
Further, as rolls with extremely improved wear resistance, WC-type cemented carbide rolls produced by sintering materials containing WC and Co are used in the forms of assembled rolls. However, these rolls are expensive and need special structures for assembling. In addition, they are poor in toughness. Accordingly, they are not necessarily advantageous except for special purposes such as finish-rolling of wires.
In the rolls, a higher wear resistance is increasingly demanded, and compound rolls provided with shell portions made of alloy powders were recently proposed.
For instance, Japanese Patent Laid-Open No. 62-7802 discloses a compound roll constituted by a shell portion and a roll core, the shell portion being made from powder of high-speed steels such as SKH52, SKH10, SKH57, SKD11, etc., high-Mo cast iron, high-Cr cast iron, high-alloy grain cast iron, Ni-Cr base alloy, etc., and diffusion-bonded to the roll core by a HIP treatment.
Japanese Patent Laid-Open No. 63-33108 discloses a roll having a roll body portion whose surface is coated with a metal-ceramic composite material by a welding method, the metal-ceramic composite material comprising a metal matrix such as Fe-base heat-resistant alloys such as Cr-Fe, Cr-Ni-Fe, Cr-Ni-Co-Fe, etc., Co-base alloys such as Cr-Co, Cr-Ni-Co, etc., and Ni-base alloys such as Cr-Ni, Cr-Co-Ni, etc. and ceramic particles of WC, Cr.sub.3 C.sub.2, CrC, SiC, TiC, Si.sub.3 N.sub.4, ZrO.sub.2, Al.sub.2 O.sub.3, etc.
These rolls show improved wear resistance as compared with the conventional cast iron rolls and forged steel rolls. However, in view of the recent demand for increased wear resistance, these rolls are still insufficient.
It is expected that wear resistance can be improved by adding large amounts of carbide-forming elements to a roll material, thereby forming large amounts of high-hardness metal carbides in the roll matrix. Particularly, since vanadium carbide (VC) shows significantly higher hardness than the other metal carbides, the wear resistance of the roll can be remarkably improved by forming VC in the roll matrix.
However, mere addition of a large amount of V to the roll material results in cast rolls in which fine carbides are not precipitated, and the distribution of the precipitated carbides is not uniform. Accordingly, such cast rolls are not satisfactory from the aspect of wear resistance and resistance to surface roughening. In addition, the larger amount of V makes casting and working of the rolls more difficult.
For instance, Japanese Patent Publication No. 42-23706 discloses a cast iron containing C, Si, Ni, Co, Cr, Mo, W, V and Mn and having excellent wear resistance, in which the amount of V is 1-6%. When the amount of V exceeds 6%, castability becomes low, and the resulting alloy becomes brittle. Since the amount of V is as low as 6% or less, the cast alloy having the above composition fails to show wear resistance on the level required in hot and cold rolls.
Japanese Patent Laid-Open No. 58-87249 discloses a wear-resistant cast roll for hot strip mill having a composition consisting essentially of 2.4-3.5% of C, 0.5-1.3% of Si, 0.3-0.8% of Mn, 0-3% of Ni, 2-7% of Cr, 2-9% of Mo, 0-10% of W, 6-14% of V, 0-4% of Co, and balance Fe and inevitable impurities. Since the roll material having the above composition contains a relatively large amount of V whose upper limit is 14%, a large amount of VC is precipitated in the roll matrix, thereby providing the roll with excellent wear resistance. However, since this roll material is produced by casting, it still suffers from the problems that the particle size of VC is not sufficiently small, and that the distribution of VC is not satisfactorily uniform.